Why Do Some Flathead Engines Use Pushrods and Rocker Arms Instead Of Tappets?

#1
I thought one of the advantages a flathead could boast against overhead valve arrangements was their ultra simplicity, specifically their lack of valvetrain components. But Indian flathead v-twins used pushrods and rocker arms instead of tappets--why?

Further, is a flathead engine using pushrods and rocker arms going to be a better performer than a flathead using tappets?

I really like old school engine swaps, and lately have been contemplating going flathead for a special project.
 

Comanche Scott

Expedition Leader
#2
Good questions, but no simple answer.

This sounds so simple, but the deeper the understanding of how an engine works, the more complicated those questions are.
A simple post isn't going to answer your question near to any acceptable level.
The "easy out" answer is; "It depends".

Your questions fill entire lifetimes of engineering study.
Today we still have the same design questions. We've designed all kinds of different valve arrangements, and even engines that don't use valves.

Ask two engineers what the best valve train system is, and you'll get at least three different answers.

The good news on these old flathead engines is, that there is a lot of excellent information on pretty much any Flathead engine out there.
So, if I were in your shoes, I'd look for the engine that best suits the purpose of your build, regardless of valve arrangement.
 

Haf-E

Expedition Leader
#4
I don't think that the engines with the rocker arms are "true" flathead engines - they are an in between design which was an attempt to improve the performance of the engine by allowing larger valves to be fitted since one is in the traditional flathead position (in the block) and the other valve is in the head.

Here is an excellent summary of the idea - they call it an "F" head engine...

http://www.curbsideclassic.com/automotive-histories/automotive-history-the-curious-f-head-engine/
 
#6
Whats more puzling to me is why doesnt anyone use these rotary valves which have been around for a couple decades at least..

http://www.coatesengine.com/csrv-advantages.html


I completely agree. The Coates technology seems amazing. Not quite revolutionary, but very close.

At this point my guess is that it may simply not be cost effective in comparison to a traditional poppet valve configuration. Which, admittedly, seems odd at first glance, seeing as how a Coates cylinder head has no poppet valves, pushrods, camshafts, springs, retainers, clips, etc, etc; eliminating all that valvetrain stuff would seem to make manufacture of a Coates engine much more affordable. Perhaps the numbers don't add up because a Coates head requires unusually expensive machining and/or materials?
 
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